332 Kirkwood : Pollen-tube in Cucurbitaceae 



not immediately apparent, only those parts of the stigma upon 

 which the pollen germinated showing any change. Under normal 

 conditions the pistillate flowers of Mclothria are pollinated early 

 in the morning from staminate flowers which open the same day. 

 The staminate flowers are very ephemeral and fall off usually in a 



few hours. - * 



The effects of pollination and fertilization in the Cucurbitaceae 

 have been carefully studied by Massart, 10 who experimented with 

 Cucurbita, Bryonia, Ecballium and TJiladiantha. He finds that the 

 excitation which determines the survival and the beginning of the 



Pep 



pollen itself and may be replaced by a traumatism ; but that the 

 excitation which provokes the general increase in the fruit proceeds 

 only from fertilized ovules. The placentae, however, do not 

 develop except in the vicinity of the fertilized ovules. 



As to the time required for the pollen-tube to reach the 

 embryo-sac, considerable variation was observed, the time vary- 

 ing twenty hours or more, depending on the number and position 

 of the ovules. After twenty-six hours the pollen-tube was 

 observed in the nucellus of Melotkria and the same condition 

 appeared in Micrampclis after nineteen hours from the time of 

 pollination. In Cyclanthcra no tubes were observed at the ovules 

 until forty-one hours had elapsed. Most of the distance between 

 the stigma and the embryo-sac is traversed by the pollen -tube in 

 three or four hours. During this time the tube has passed 

 through the style and into the ovary. The growth is much 

 slower as the tube nears the micropyle. This feature seems to 

 be correlated with the amount of available reserve food in the 

 tube, which is of course greatest at the beginning of its growth. 



In the Cucurbitaceae the form and structure of the ovary in 

 different genera varies a good deal, but in most cases the cavity 

 of the ovary is practically filled by the fleshy development of the 

 placentae and the pericarp. A fusion of the ingrowing lobes of 

 the placenta often occurs {Cucurbita, Citrullus) and leaves no space 

 between them. In a similar way the stylar canal may also be 

 obliterated. In Melotkria, Micrampclis and Cyclanthcra, however, 

 such fusion does not take place before fertilization. The structure 

 of the ovary in Mclothria is similar to that of Cucurbita, except 



